This invention relates to a plastics storage tank, and in particular to a tank made of thermoplastic material for use as an individual tank or a tank battery for the storage of, for example, fuel oil.
The invention is concerned in particular with plastics tanks made by blow-moulding and having large dimensions and a correspondingly large capacity, generally of at least 1000 liters.
It is well known that plastics tanks, which are used as individual tanks or tank bateries for the purpose of storing fuel oil and the like, must be strengthened by means of steel straps so as to prevent undesirable bulging of the walls of the tanks under the effect of the static pressure of the fluid contents. The provision and fitting of the steel straps involves considerable additional cost.
For many years it has been known to increase the stability of storage tanks by providing, between the walls of the tanks, reinforcing webs or the like which act as tension flanges. In this connection it has also been proposed, in the production of storage tanks by the blow-moulding process, to fit special prefabricated plastics structural elements in the tank so as to brace and strengthen the tank walls (German Patent Specification OS1947737). In this way, the structural elements have to be introduced into a tube-like plastics blank while the mould is still open and they have to be aligned along the planes in which they are to be secured before the mould is closed and the plastics blank is brought to its final shape by blowing.
It is also known to strengthen the walls of tanks by forming integrally with two of the side walls of the tank substantially funnel-shaped recessed portions which are welded to each other by their base surfaces at a zone located roughly at the centre of the interior of the tank. Although this step results in a reduction in the bulging of the walls of the tank so that the use of special straps can be dispensed with, it has the disadvantage that a substantial part of the capacity of the tank is sacrificed by having to accommodate the funnel-shaped recessed wall portions. In order to offset this loss of capacity, the tank body has to be of considerably larger external dimensions. This leads, among other things, to increased material costs which in turn nullify the savings resulting from the absence of straps. Furthermore, the greater size of these storage tanks causes very considerable disadvantages as regards transportation, stocking and the installation of tank bateries in what are usually constricted spaces.
The main aim of the invention is to provide a large-capacity tank, produced by the blow-moulding method, as an individual tank or tank battery for the storage of fuel oil or other fluids, the walls of which tank are strengthened to such an extent by integral reinforcing means, without however reducing its capacity to an unacceptable degree, that the tank can be used without the addition of costly steel straps and the like. The invention is also concerned with an advantageous and economical method of producing such reinforced plastics tanks by blow-moulding.